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Composites of epoxy resin with high percentage of silica fillers (66%) are designed to improve mechanical and electrical properties of transformers used in railway application. FTIR, (DRX and FE-SEM) and absorption/desorption phenomena are used to investigate the structure, the morphology and the diffusion of water in the microcomposites, respectively. Good dispersion of silica filler with size less than 10 μm was assessed by SEM, although some clustering (agglomerates) of greater than 1 μm was observed. The absorption water in the microcomposite obeys to the first Fickian law and shows saturation water of 0.6%. The calculation of the diffusion coefficient of water leads to a value of 2.9*10 −12 m 2 s −1 in the studied system. Relaxation times of α -relaxation and ionic conduction relaxation processes are determined. A correlation is observed between the ionic conductivity and dielectric relaxation processes. The dc-current behavior shows a change in the conduction mechanism from electronic conduction below the T g to ionic conduction above the T g . Shallow traps of 0.54 eV and deep traps of 2.21 eV are determined below and above the T g , respectively. The TSC analysis confirms the VFT behavior of the α -relaxation of the microcomposite as obtained by the dielectric spectroscopy. [ABSTRACT FROM AUTHOR]